1. Field of the Invention
The present invention relates to a method and apparatus for the alignment of optical components and particularly, but not exclusively, for the alignment of an optical fibre to an integrated optical component. The invention finds particular application in the alignment of an optical fibre with an integrated waveguide component comprising a substrate having an integral waveguide or further optical elements located in or on the substrate. Optical arrangements comprising optical fibres and integrated waveguide components are common in, for example, optical modulators for high-speed telecommunications.
2. Discussion of Prior Art
When mounting an optical element such as an optical fibre to an optical waveguide, and particularly to an integrated waveguide component (hereafter referred to as an IWC) comprising a substrate having an integral waveguide, it is necessary to substantially exactly align the optical fibre with the waveguide so as to minimise signal loss between the optical fibre and the waveguide.
There are currently two main techniques used by manufacturers to align optical fibres with IWCs. The first known method involves manually manipulating the optical fibre and a coupling lens arrangement until light from the optical fibre is focussed by the lens arrangement onto the input face or facet of the IWC.
The second known method involves manually aligning and butting the optical fibre to the input facet of the IWC.
In the above methods, an optical detector is placed at the output facet of the IWC to image or monitor the light passing through the waveguide. A particular waveguide “mode” signal is produced when the optical fibre is substantially exactly aligned with the waveguide and it is this mode signal which the detector is arranged to detect. When the optical fibre is substantially exactly aligned with the waveguide in the IWC, the waveguide is excited and the mode signal is produced. The optical fibre is then fixed in place relative to the IWC by any appropriate means.
Both of the aforementioned methods do, however, exhibit considerable disadvantages. Firstly, since the waveguide is relatively narrow, it takes a considerable time to manually manipulate the optical fibre to a position where the waveguide mode is excited and the waveguide mode signal is produced. Secondly, because the light from the optical fibre is diffracted as it emerges from the end of the fibre, some of the light can scatter into the substrate of the IWC and is thus transmitted through the substrate (i.e. not through the waveguide proper). This diffracted light is also received by the detector and interferes with the mode signal produced by the waveguide, significantly hindering the detector's ability to detect the mode signal. As a result, a relatively long time can be required to align the optical fibre with the waveguide, typically approaching one hour, using these processes. Moreover, the process generally requires manual manipulation and cannot be achieved with any significant degree of automation.
An alternative technique for minimising signal loss when coupling an optical signal between an optical element, such as an optical fibre, and an optical waveguide is described in GB 2,368,658. GB 2,368,658 utilises an optical taper located between the optical element and the optical waveguide to funnel light there-between. Consequently, the optical element and the optical waveguide do not need to be precisely aligned to achieve efficient coupling of an optical signal between the two optical elements. Thus, the inclusion of the optical taper has the effect of relaxing the tolerance required for aligning the optical element and the optical waveguide.
Whilst the technique described in GB 2,368,658 may allow signal losses to be minimised, the technique may not be applicable to applications requiring single mode excitation in the waveguide. The optical taper potentially disrupts the collection angle of the waveguide, resulting in multi-mode excitation within the waveguide rather than single mode excitation.
It would be advantageous to provide an improved method and apparatus for facilitating the alignment of the optical fibre with the waveguide, which alleviates the aforementioned difficulties and permits a degree of automation to be involved. It is an aim of the present invention to provide such a method and apparatus.